Fatigue strength is presented by means of the so-called S-N curve which is experimentally determined by straining the test samples at different strain or stress amplitudes till the final fatigue fracture. One of the most important detail of the S-N curve is the endurance or fatigue limit, i.e. the limiting strain or stress amplitude under which the applied amplitudes are no longer capable of causing fatigue fracture or destruction of the samples. From this point of view of practical applications, the endurance or fatigue limit is also the most important value of the fatigue strength. Determination of this limit in the above mentioned destructive method requires a period of 5 to 10 days and nights depending on the testing machine used.
There exist also some non-destructive methods to measure fatigue tendency, mainly to follow the development of the structure deterioration associated with fatigue. A characteristic of these methods is that they require a comparison of the measured results with those of earlier measurements or those of reference materials. These methods, however, are proven to be rather unreliable, and, furthermore, with them one cannot measure the most important fatigue strength, the endurance of fatigue limit. Examples of these methods are, among others, the fatigue gauge method (fatigue wire methods) and measurements of the coersitive force.
The present non-destructive method of measuring the endurance or fatigue limit applies the so-called mechanical Barkhausen noise phenomenon. As a phenomenon, the mechanical Barkhausen noise formation is related to the irreversible changes in the magnetic state of the material (with the discontinuous movements of the domain walls under the influence of mechanical loading of the sample in a stable magnetic field or in a demagnetised state).
Better known is the "ordinary" Barkhausen noise effect which resulted from a changing external magnetic field. For the application of this "ordinary" Barkhausen noise effect in the non-destructive measuring technique a number of devices have been developed. For example the U.S. Pat. No. 3,427,872, teaching a method for the measurement of the static loading or strain state of a material by means of a changing external magnetic field is based on this "ordinary" noise effect. This method cannot be applied, however, to the analyzing of the mechanical Barkhausen noise, and may not be applied for the determination of the endurance or fatigue limit of the material.
The method of the present invention offers the notable advantage that the endurance or fatigue limit can, in practice, be determined very quickly, for example, in a period of 15 minutes. Therefore, the method of the present invention can in practice be applied to determine immediately whether the dynamic loading of a tested machine part is too large (above fatigue limit) or not.
The invention will now be described further by way of example only and with reference to the accompanying drawings.